1. Technical Field
This invention relates to a circuit arrangement for a digital circuit using differential logic, particularly for emitter followers. Emitter followers are generally known as basic circuits, cf. Tietze, Schenk: Halbleiter-Schaltungstechnik, 8th Edition, Springer-Verlag Berlin, 1986, pages 56 to 59. This basic circuit is used to provide isolation between subcircuits or as a level shifter.
2. Discussion of Related Art
In circuit arrangements for optical communications systems operating at high data rates, for example 10 Gb/s, use is made of emitter followers in differential logic as are shown in FIG. 1 as a series combination of three switching stages 1, 2, 3. Logic signals "0" or "1" are applied to the inputs inp and inn of a switching stage in complementary form, so that the power consumption of an emitter-follower pair so driven is constant regardless of the information content of the signal to be transmitted. Such a constant power consumption is particularly advantageous at high transmission speeds, as disturbances caused by switching operations are thus minimized and, on the other hand, electromagnetic interference with the environment is greatly reduced even at high transmission speeds if a constant electromagnetic field is present.
As it is desirable to keep the heat generated in integrated circuits for optical communications systems to a minimum, there is a need for a circuit arrangement using differential logic which has a lower power consumption than prior-art circuits.
In the circuit arrangement shown in FIG. 3, showing examples of prior art component values, it would be possible to reduce the power consumption by making the resistors R1, R2, R3, R4, R5, R6 in the switching stages 1, 2, 3 larger, but this measure would result in the transistors T1, T2, T3, T4, T5, T6 being operated below the optimum current density, so that their switching speed would decrease; in addition, the voltage swing of the switching stages 1, 2, 3 would become smaller, so that the effect of noise and the sensitivity to interference would increase.